1. Field of the Invention
The invention is directed to a method for correcting image errors given a change in the operating mode, with a change in the radiation dose in the context of an x-ray diagnostic measurement, the method being of the type which makes use of the memory effect of a radiation-sensitive structure, particularly a photodiode array of a solid-state detector which receives the radiation, and wherein a correction image is obtained that is subtracted from the detector image obtained with the solid-state detector. The invention is also directed to an x-ray diagnostic apparatus and to a solid-state radiation detector operating according to the method.
2. Description of the Prior Art
Solid-state detectors are replacing the previously widely used film/screen systems. X-ray image intensifier/video chain systems are also currently utilized to an increasing extent in x-ray diagnostics. Such solid-state detectors, as employed, for example, in an image pick-up arrangement disclosed in PCT Application WO 96/16510, are constructed on the basis of a semiconductor component in which the radiation-sensitive structure is fashioned dependent on the type of solid-state detector. An exemplary embodiment of such a solid-state detector is photodiode matrix of amorphous silicon. The functioning of this solid-state detector is based on the generation of charge carriers when radiation is incident on one of the photodiodes fashioned in the semiconductor, the charge carriers being dependent on the type and intensity of the incident radiation. Since the photodiodes are usually sensitive only in a visible wavelength range, a scintillation layer, usually a cesium iodide layer, precedes the photodiodes for the conversion of the x-ray quanta, for which the photodiode is insensitive, into visible light.
The employment of such solid-state detectors has a number of advantages compared to the previously used systems, for example a small structural size and the low weight. Further, no high-voltage elements and phases are required; the power consumption is also lower. Such detectors also exhibit more significant contrast properties, but without the usual geometrical distortions arising therefrom.
A characteristic known as the memory effect, however, represents a significant problem associated with solid-state detectors. This effect is caused by charge carrier traps (often simply referred to as "traps") which are unavoidably formed in the semiconductor material and which capture a part of the charge carriers generated by the radiation exposure, and in turn release the trapped charge carriers over time due to energy activation, for example as a consequence of thermal events. This means that the charge carriers generated by a preceding exposure are not completely removed (or reset) in the following read-out cycle; on the contrary, a number of charge carriers remain trapped in the traps. These are only gradually removed in later read-out cycles and produce what is referred as a "residual image", which represents the cumulative effect of the gradually released charge carriers from a number of cycles. This occurs with every new image in a continuous sequence, so that the effects superimpose.
The memory effect is uncritical when image sequences are registered with a constant radiation dose, and thus with an electronic read-out amplification that also remains constant, since the residual image has already decayed to approximately 3% after about 160 ms; consequently, it makes only a negligible contribution to the following images. A correction of the effect then is not required for achieving an adequate image quality. When, by contrast, a sequence is operated with high radiation dose--which means that a number of charge carriers are generated as a consequence of the high dose, and consequently a low electronic read-out amplification is adequate, and a switch is made immediately thereafter to a low radiation dose (which involves a high electronic read-out amplification in order to achieve an adequate contrast), then a visible slowly decaying residual image can be observed that overlays the image sequence given the low dose. In this case, the decaying residual image that arises from the high exposure dose can still be as strong (visible) as the useful image generated by the low radiation dose up to a few seconds following the moment of switching. A correction is then essential in order to be able to utilize the image signals obtained immediately after the switch. Such an operating mode, namely the switch from a high to a low radiation dose, is very common in practice and is employed, for example, in order first to undertake a positioning of the patient or working device at a low radiation dose with low stress on the patient, for example in order to introduce a catheter into a coronary artery. The positioning can be adequately identified in a mode known as "fluoroscopic mode." After the positioning has been completed, the exposure mode is conducted using a high radiation dose, for example upon the addition of a contrast agent. For removing the catheter or for repositioning the catheter, for example, a switch is subsequently made back into the fluoroscopic mode with low radiation.
In order to correct the image errors arising from the memory effect, European Patent Application 0 642 264, for example, discloses a method wherein a correction image is continuously subtracted from the detector image, i.e. from the supplied image signal. This correction image is determined either based on a simulation of the physical events that are responsible for the memory effect and the decay of the charge carriers, or by pure calculation. The decay behavior, and thus the residual image, however, are dependent on a multitude of factors, for example on the dose, the time duration and the point in time of the preceding x-ray shots and, of course, also on the type of solid-state detector employed. Thus a correction using the method described in European Patent Application 0 642 264 is possible only with difficulty, particularly in the extreme range when switching the operating mode.